Hard disk drives are used in almost all computer system operations. In fact, most computing systems are not operational without some type of hard disk drive to store the most basic computing information such as the boot operation, the operating system, the applications, and the like. In general, the hard disk drive is a device which may or may not be removable, but without which the computing system will generally not operate.
The basic hard disk drive model was established approximately 50 years ago and resembles a phonograph. That is, the hard drive model includes a storage disk or hard disk that spins at a substantially constant rotational speed. An actuator arm or slider is utilized to reach out over the disk. The arm has a head-gimbal-assembly (HGA) composed of a suspension, flexure and a slider carrying the read/write components.
In operation, the hard disk is rotated at a set speed via a spindle motor assembly having a central drive hub. Additionally, there are tracks evenly spaced at known intervals across the disk. When a request for a read of a specific portion or track is received, the actuator and servo-system of the hard drive aligns the head, via the arm, over the specific track location and the head reads the information from the disk. In the same manner, when a request for a write of a specific portion or track is received, the hard disk aligns the head, via the arm, over the specific track location and the head writes the information to the disk.
One phenomenon, known as “stick slip,” may occur in hard disk drives which can cause a significant displacement of the actuator arm. This phenomenon is particularly problematic when the hard disk drive is exposed to a thermal gradient. The thermal gradient may be the result of, for example, a hard disk drive in a laptop computer being moved from a colder environment into a warmer environment. Alternatively, a hard disk drive in computer which has been turned off for an extended period may experience a significant thermal gradient when the computer is turned on again.
FIG. 1 is a cross section view showing an exemplary voice coil magnet assembly 101 mounted on a housing 102 of a hard disk drive. As the hard disk drive begins to warm up, the voice coil magnet assembly 101 and housing 102 expand at different rates in accordance with the thermal coefficients of the materials of which they are made. Because the components are tightly coupled during the fabrication process by screw 103, movement of voice coil magnet assembly 101 relative to housing 102 is constrained due to friction caused by the clamping force applied by screw 103. Thus, stress builds where voice coil magnet assembly 101 is in contact with housing 102. As the two components continue to expand at different rates, the stress becomes greater than the friction between the components and the stick slip event occurs in a manner similar to an earthquake relieving pressure between two tectonic plates.
This is shown in FIG. 2 which shows the outputs of accelerometers which were disposed on a hard disk drive which was exposed to a thermal gradient. At a plurality of events (e.g., 201, 202, 203, and 204) the relief of stress between voice coil magnet assembly 101 and housing 102 (e.g., a stick slip event) is of sufficient magnitude to be detected by the accelerometers.
When a stick slip event occurs between voice coil magnet assembly 101 and housing 102, the actuator arm may be displaced enough to exceed the limits of the position error signal (PES) parameters for the hard disk drive. Typically when this occurs, further writing to the hard disk is inhibited by the servo mechanism of the hard disk drive, or an off-track write error. Alternatively, the actuator arm may be displaced to an entirely different data track. When this occurs, writing to the disk may continue until an off-track write error is recorded. When this happens, writing to the hard disk is suspended, the actuator arm is re-positioned over the correct data track, and writing to the hard disk is resumed. However, until the writing to the hard disk is stopped, previously saved data may be overwritten.